Attenuation is an inelastic process that dissipates energy by conversion of acoustic energy into heat, thus decreasing wave amplitude and modifying the frequency and phase content of a propagating wavelet. Most of the measurements, processing, and interpretation efforts in the industry and in academia have concentrated on wave velocity rather than attenuation data since velocity measurements are easier to conduct, more reliable and efficient. There is a growing interest recently in attenuation measurements. The frequency content, phase spectrum, and velocity can be used as strong indicators of the type of pore fluid in a formation. Presently in the art, attenuation could only be determined using core samples and it would be impossible to obtain such calculations in real time while drilling.
Ultrasonic attenuation measurements using core samples are known. For example, U.S. Pat. No. 4,380,930 discloses a system for transmitting ultrasonic energy through a material sample which includes an ultrasonic energy transducing means in contact with said material sample for transmitting ultrasonic energy into said sample and for receiving the energy after it has traveled through said sample, a pressure cell for housing said sample under a confining pressure simulating subterranean pressure conditions, and means for isolating said ultrasonic energy transducing means from the confining pressure conditions on said cell such that said ultrasonic energy transducing means operates at ambient pressure conditions.
U.S. Pat. No. 4,631,963 discloses a method for measuring acoustic energy anisotropy of a core sample from a subterranean formation wherein the sample is shaped to provide a plurality of pairs of parallel, planar outer surfaces about the length of the core sample, and acoustic travel time, attenuation, waveform or other acoustic properties are measured through said core sample in each of the azimuthal directions through said core sample which are perpendicular to each of said pairs of parallel, planar outer surfaces, and each of said measured acoustic properties are compared to identify the azimuthal direction of any acoustic energy anisotropy through said core sample. Also see U.S. Pat. No. 4,631,964.
Cuttings produced during drilling represent a potential quasi-real time source of information that can be procured at the rig site. The use of cuttings has been limited in the past, partly due to the difficulties in performing measurements on very small samples and obtaining any kind of accurate results. Drill cuttings could potentially provide an invaluable source of information for calibrating seismic velocities, logging while drilling (LWD) and/or wireline sonic measurements.
Continuous Wave Technique (Hereafter CWT) has been used in the past for measurement of acoustic phase velocities using core samples. This methodology has been recently utilized in a tool, CWT equipment, that is particularly well suited for testing of small samples like drill cuttings, and measurements on sub-inch shale cuttings.
In “Rig-site and Laboratory use of CWT Acoustic Velocity Measurements on Cuttings”, by Nes, et al, Society of Petroleum Engineers Paper No. 36854, 1996, incorporated by reference herein in the entirety, there is presented the use of continuous wave technology (CWT) for measurement of acoustic phase velocities on cuttings using potentially portable equipment that is suitable for testing of small samples of cuttings, thus offering a new source of data that can be attained in quasi real-time at the rig site.
There is a great need in the art for a method that would make it possible to accurately predict velocity and attenuation in real time measurements at the rig, site in various lithologies including shales. If such data were available it would permit calibration of seismic velocities and sonic log velocities and improve our fluid and pressure monitoring capabilities in real time, resulting in significant savings during drilling and production, and would also enhance our reserve estimates.